Light emitting diodes (LEDs) have gained popularity for use in general illumination because of their very long life and relatively low operating cost in comparison to conventional incandescent lighting. An array of LEDs can produce light intensity sufficient to replace an MR-16 incandescent lamp or an equivalent fluorescent lamp. Due to their small size, LEDs can be arranged in fairly dense arrays to produce a significant amount of light per area, especially when multiple die and/or high intensity LEDs are used.
High power and high density LEDs produce large amounts of heat along with the high light output produced. As the density of the LEDs increase, the amount of heat dissipation needed also increases. LEDs are extremely sensitive to operating temperatures. High temperatures can reduce the light output, or Lumens per Watt, and can also reduce the operating lifetime, or even destroy the LED. Because of these temperature concerns, heat dissipation devices have been developed to cool the LEDs.
Some conventional heat dissipation devices use passive systems with heat sinks made from high thermal conductivity metals, such as aluminum or copper, to move heat away from the LED to where the heat can be dissipated into the surrounding air using cooling fins or other such structures. In some applications, however, these heat sinks cannot move heat quickly enough from the local area of the die because the amount of heat produced by multi-die and other high power LEDs is more than can be removed with a heat sink that is reasonably small enough to be included in a LED lighting product. Moreover, when the pure metals characterized by high thermal conductivity are alloyed with other metals to improve machinability, or to allow casting or forging, the thermal conductivity of the alloy metal is significantly diminished.
Another heat removal system involves the use of a heat pipe. The heat pipe systems are an attractive solution to LED heat problem in that they are light weight and allow for a heat exchanger to be located remotely. Moreover, the thermal conductivity of these systems can be as high as metals because they rely on the transition between liquid and vapor and the enthalpy of transition is high for liquids such as water. However, the heat capacity of vapor based heat pipes is low and poses a limitation on the amount of heat that can be removed since the transport of the liquid and the amount of liquid in the system present an upper limit to the amount of heat that can be transported and removed. Since only a small volume of liquid can be accommodated (usually a few cc's), the total amount of heat that can be moved is low. Furthermore, the cost of fabrication of a heat pipe system can make the system cost prohibitive.
Convection can be used to remove heat from the LED in some instances. However, since convection relies on gravity to work, the LED must be oriented so that the convection heat path is up from the LED location to move the heat away from the LED. Since lighting products must operate in a variety of orientations, conventional convection heat removal is not always the best solution. In addition, traditional convection uses air to carry the heat. Air has a relatively low heat capacity and therefore cannot remove heat rapidly unless impractically large volumes of air are used.
Any cost effective method that lowers the temperature of the LED during operation will improve the efficiency of the light device, provided it does not consume the power gained in the process. A fan would have to be utilized in order to move enough air to remove the heat from the LEDs using air for convection. Fans, like other active cooling methods, draw energy and reduce the efficiency of the light device. In addition, fans do not have the operating lifetime of a LED which can be from 50 to 100 k hours. Fans also create noise, which is an unnecessary distraction that a lighting device can do without.
Conventional liquid cooling can also be used and also has some beneficial attributes. One benefit is that liquid has a higher thermal conductivity that air and so can carry heat away from the LED with much greater efficiency. However, conventional liquid cooling systems use pumping which adds additional cost and energy usage and decreases the overall operating lifetime and efficiency of the lighting device because of the mechanical pump.
The present invention provides a highly advantageous LED cooling device and method that are submitted to resolve the foregoing problems and concerns while providing still further advantages, as described hereinafter.